Capturing the position and the alignment of a movable part of a coordinate measuring machine on the basis of an optical capture of markers is known from, e.g., DE 10 2015 205 738 A1. Like in an exemplary embodiment of the disclosure, too, the position of the movable part and/or alignment thereof can be used to control or regulate the movement of the movable part.
In particular, the disclosure includes tracking the movement of the target body in order to determine a number of local degrees of freedom of the movement. The movement tracking contains, in particular, a repeated capture of the target body such that the position, the speed and/or the alignment of said target body is/are ascertainable in each case from the result of a capture of the target body, optionally taking account of capture results obtained from an earlier movement state and/or a subsequent movement state. The capture may in particular include recording one or more images of the target body, for example by one or more digital cameras.
In general, a movement state of the target body, in particular the current position, current alignment and/or current speed, can be determined with regard to a number of spatial degrees of freedom of the movement of a tool by evaluating the captured information items, i.e., the information items that are/were obtained during the capture process. By way of example, the tool can be a tactile probe or a sensor, a non-tactile, in particular optical, sensor for determining coordinates of a workpiece, a processing tool (e.g., a mechanical, optical, chemical or other processing tool) for processing a tool, or a tool for adding material to a workpiece. Independently of the specific application, it is often the object to initially ascertain the position and/or alignment of the target body, and hence, in particular, the position and/or alignment of the tool, from the capture.
The number of degrees of freedom of movement to be captured or the number of captured degrees of freedom of movement, and hence also the coordinate axes and/or axes of rotation, with regard to which the position and/or alignment should be established or is established, may be of different magnitude. By way of example, the position can be determined with regard to one, two or three linear axes of the movement or coordinate axes. As an alternative or in addition thereto, the alignment can be ascertained with regard to one, two or three axes of rotation and/or as an alignment vector in a two-dimensional or three-dimensional coordinate system.
If the target body is captured by at least one image, in particular a digital image, methods of image processing known per se can be resorted to when evaluating the captured information items. In general, the ascertainment of the position and/or alignment of the object (e.g., the tool) actually under observation requires the ability to uniquely identify the object or an object connected therewith or a combination of the two objects. Consequently, the identification must be reliable and should be implemented quickly in most applications. If a plurality of objects should be observed simultaneously and, in particular, if the movement thereof should be tracked, the objects must also be uniquely identifiable, or at least distinguishable from one another.
Therefore, the use of so-called targets, which are combined with the actual object to be observed and which are fastened to the latter, for example, is known. The targets can be configured in such a way that they are quickly capturable in a reliable and distinguishable manner. The distinguishability relates not only to different movable objects, but also to a single, or each individual, movable object, which should be distinguishable from its surroundings and its background. Suitably designed targets can also ensure this distinguishability is satisfied.
Particularly when controlling movement processes that are based on so-called movement tracking, i.e., which use, e.g., the position and/or alignment ascertained from the captured information items as a basis for the control, both the capture and the evaluation of the capture information items should be robust, i.e., the susceptibility to errors should be low. Errors may lead to erroneous measurement results, incorrectly processed workpieces, incorrectly produced workpieces and collisions. In medical therapy, corresponding treatment errors may arise in the case of capture errors. This applies if, for example, the treatment is carried out at least in part by a robot.
The preceding and the following description also relate, in particular, to the disclosure and configurations thereof.
As mentioned at the outset, targets may have a plurality of markers which together form the target for an optical capture. Optical is understood to mean that it is electromagnetic radiation that is captured, said electromagnetic radiation, in particular, corresponding to the movement state of the target. Areal markers, i.e., markers that extend along an area, in particular a surface, are already known per se. For areal markers, marker structures extending transversely to the area are not important for the capture in this case. By way of example, such areal markers can be realized as two-dimensional grayscale value distributions or binary distributions. In particular, binary distributions have dark and bright regions, e.g., black and white regions. Examples include one-dimensional barcodes and two-dimensional matrix codes. Areal markers, in particular two-dimensional markers, can be, e.g., printed onto a surface (e.g., by inkjet printing), etched into the surface and/or introduced into the surface in any other way by partial material removal and/or applied by material application. Expressed differently, the surface can be structured in such a way that an aerial marker arises. During the application, a continuous layer with different optical properties can be produced, for example, and/or material can be applied in portions of the surface only.
The markers can be optimized for the respective application. By way of example, they may contain a code or may be combined with a code in order to make these distinguishable from other markers. The markers and hence the targets should also be optimized for determining the position and/or alignment of the target and for specific applications such as the ascertainment of the movement speed.
If the capture information items are two-dimensional information items, as is the case for digital camera images, for example, then it is possible to refer to a viewing direction, in particular the optical axis of the camera, which extends perpendicular to the two-dimensional area of the capture information items. Areal markers whose alignment is not equal to the viewing direction can be considered to be rotated about an axis of rotation extending perpendicular to the viewing direction. A problem arising here is that the same distortion or deformation of the appearance of the marker arises if the marker is rotated from the viewing direction in one rotational direction or in the opposite rotational direction about the aforementioned axis of rotation. Which of two possible alignments the marker has is not ascertainable without additional information items. A single image or, formulated more generally, local two-dimensional capture information items is/are insufficient in this respect.